1. Field of the Invention
The present invention relates to a thermal-assist magnetic recording medium used in a hard disk device (HDD) and the like and a magnetic recording and reproducing apparatus.
Priority is claimed on Japanese Patent Application No. 2008-306653, filed Dec. 1, 2008, the content of which is incorporated herein by reference.
2. Description of the Related Art
With the recent rapid development of the information-oriented society, an additional increase in capacity has been required for a hard disk device (HDD). To enhance a surface recording density of a magnetic recording medium used in the HDD, it is necessary to reduce the particle size of magnetic crystal grains. However, the decrease in particle size of magnetic particles causes a problem with the deterioration in thermal stability.
In general, KuV/kT (Ku: crystalline magnetic anisotropic constant, V: magnetic particle volume, k: Boltzmann constant, and T: absolute temperature) is used as an indicator indicating the thermal stability. To guarantee satisfactory thermal stability, the value of KuV/kT needs to be equal to or greater than 60.
Since the decrease in size of magnetic particles causes the decrease of V, the value of KuV/kT decreases, thereby deteriorating the thermal stability. To prevent this deterioration in thermal stability, it is necessary to enhance the value of Ku. However, the enhancement in Ku causes an increase in anisotropy magnetic field Hk. This is because the relation of Ku=(Ms×Hk)/2 is established. Accordingly, when the value of Hk is greater than the recording magnetic field of a recording head, a writing operation is not satisfactorily performed and it is thus necessary to set Hk to be lower than the recording magnetic field. This serves as a factor for determining the upper limit of Ku, that is, the lower limit of the decrease in particle size.
The recording magnetic field Hw of an existing recording head is in the range of about 10 to 12 kOe. Hw is determined by the magnetization of a magnetic material used in a magnetic pole, but the magnetization of a FeCo alloy used in the magnetic pole of the existing recording head is in the range of about 2.2 to 2.4 T. This is almost the same as the maximum value of the magnetization of the FeCo alloy indicated by a Slater-Pauling curve. Accordingly, it is difficult to enhance the value of Hw greatly from the existing level. Therefore, in consideration of KuV/kT>60, the limit of the decrease in size of the magnetic particles is about 10 nm.
As a technique for solving the limit, a magnetic recording medium called exchange coupled composite (ECC) media was suggested. This is a magnetic recording medium in which a magnetic layer includes a magnetically-hard recording layer (hard layer) and a soft magnetic layer (soft layer) formed thereon.
A thin coupling control layer for properly controlling the interlayer coupling between the hard layer and the soft layer is formed therebetween. By optimizing the thickness of the coupling control layer, the inverting magnetic field of the recording layer (hard layer) can be reduced up to a half or less of the case where the soft layer is not provided. Accordingly, it is possible to increase Hk (increase in Ku), thereby further decreasing the particle size.
In the ECC media, the magnetic particles can be reduced in size up to 6 to 7 nm, in consideration of the recording magnetic field and the thermal stability (Hw=10 to 12 kOe, KuV/kT>60) as described above. However, to embody the surface recording density of 1 Tbit/inch2, it is necessary to reduce the size of the magnetic particles up to 6 nm or less and it is thus difficult to embody the technique in the ECC media.
As a technique for accomplishing the surface recording density of 1 Tbit/inch2 or more, thermal-assist recording has attracted attention. The thermal-assist recording is a recording technique of applying near-field light as a laser beam to a medium, locally heating the medium to reduce the coercive force of the medium, and performing a writing operation.
In this case, a writing operation can be performed using the recording magnetic field of the existing head even on a recording medium of which the coercive force at the room temperature is several tens kOe. Accordingly, by forming a recording layer of a material with high Ku of 106 J/m3, it is possible to reduce the size of the magnetic particles up to 6 nm or less with the thermal stability maintained. As such a high-Ku material, a FePt alloy (of which Ku is about 7×106 J/m3) or a CoPt alloy (of which Ku is about 5×106 J/m3) having an L10-type crystal structure was known.
In the thermal-assist recording, to reduce a magnetization transition width to obtain excellent recording and reproducing characteristics, it is necessary to make the temperature gradient of the coercive force as rapid as possible. In general, the temperature gradient of Hc is the rapidest in the vicinity of the Curie temperature. Accordingly, by performing the writing operation after heating the medium up to the vicinity of the Curie temperature, it is possible to reduce the magnetization transition width.
However, for example, in the L10-type FePt alloy, the Curie temperature is 487° C., which is much greater than the heatproof temperature (about 350° C.) of an HDD glass substrate used widely or the decomposition temperature (about 400° C.) of lubricant. Accordingly, it is difficult to heat the medium up to the vicinity of the Curie temperature.
As a technique for enhancing the temperature gradient of the coercive force at a temperature lower than the heatproof temperature of the glass substrate, a medium in which a FeRh alloy with a B2 structure and a FePt alloy with the L10-type structure are stacked was suggested (Non-Patent Document 1: Appl. Phys. Lett., Vol. 82, pp. 2859-2861 (2003)). It was known that the FeRh alloy with the B2 structure is transited in phase from antiferromagnetism to ferromagnetism at a transition temperature in the vicinity of 90° C. Accordingly, when the medium is heated up to the vicinity of the transition temperature, the magnetization is caused to assist the magnetization inversion of FePt, thereby rapidly decreasing the coercive force.
It is described in Non-Patent Document 1 that the transition temperature of the FeRh alloy can be enhanced to 200° C. by adding Ir of 3 at%. Accordingly, it is possible to embody the temperature dependency of Hc rapid in the range of 90° C. to 200° C. which is lower than the heatproof temperature of the glass substrate.
As another technique for increasing the temperature gradient of the coercive force at a low temperature, a medium having a structure in which a magnetic layer (switching layer) having a low Curie temperature is interposed between two magnetic layers (a recording layer and a base layer) having a high Curie temperature is disclosed (Patent Document 1: JP-A-2002-358616). Patent Document 1 also describes that the exchange coupling of the recording layer and the base layer disappears by heating the medium up to the Curie temperature or higher of the switching layer, whereby Ku is rapidly decreased with the temperature.
With the same layer structure, a medium in which a CoCrPt—SiO2 recording layer is formed on a L10-type FePt layer having very high Ku with a γ-FeMn antiferromagnetic layer having a block temperature of 155° C. interposed therebetween is disclosed (see Patent Document 2: JP-A-2007-59008). Patent Document 2 also describes that the exchange coupling of the CoCrPt—SiO2 recording layer and the FePt layer disappears by heating the medium up to the block temperature or higher of the γ-FeMn, whereby He can be rapidly decreased with the temperature.
A technique of making the temperature gradient of He rapider by stacking a ferromagnetic recording layer and an antiferromagnetic layer is disclosed (see Patent Document 3: JP-A-2008-52869). Patent Document 3 also describes that the coupling of the ferromagnetic recording layer and the antiferromagnetic layer is made to disappear by heating the medium up to the blocking temperature or higher of the antiferromagnetic layer, whereby Hc is rapidly decreased.
As described above, since the L10-type FePt alloy or the CoPt alloy having high crystal magnetic anisotropic Ku can be decreased in particle size with the satisfactory thermal stability maintained, such alloys are suitable for the magnetic layer of the thermal-assist magnetic recording medium.
However, to regularize the alloys to obtain the L10-type crystal structure, it is necessary to heat the substrate at 500° C. or higher or to perform a heating process after forming the layers. Since the temperature is much higher than the heatproof temperature (about 350° C.) of the HDD glass substrate, it is difficult to embody such a structure.
As a countermeasure thereof, it was reported that the regularization temperature can be lowered by adding Ag or Cu to the FePt alloy. However, when the regularization temperature is lowered, the value of Ku is also greatly decreased and thus both have a trade-off relation.
In the thermal-assist recording, to obtain excellent recording and reproducing characteristics, it is necessary to reduce the exchange interaction between the magnetic particles satisfactorily. Accordingly, a medium employing a magnetic layer obtained by adding oxide of SiO2 or carbon (C) to the L10-type FePt alloy was suggested. However, in this case, it was reported that the regularization of the L10-type crystal structure is suppressed to lower greatly the value of Ku.
In consideration of the above descriptions, it is very difficult to manufacture a magnetic layer in which the exchange interaction between the particles is sufficiently deactivated and which has a high Ku value of 2×106 J/m3 at a substrate temperature of 350° C. or less.
In case of the FeRh alloy with the B2 structure suggested as the technique for making the temperature gradient of He rapid at a low temperature equal to or lower than the heatproof temperature of the glass substrate, it is necessary to perform a heating process at a high temperature or to heat the substrate for the purpose of regularization. Accordingly, in this case, monocrystalline MgO with high heatproof is used in a substrate and the substrate is heated at 550° C. to regularize the FeRh alloy (see Non-Patent Document 2: Jpn., J. Appl. Phys., Vol 45, no. 2B, pp 1314-1320 (2006)). This temperature is much greater than the heatproof temperature of the glass substrate.
On the other hand, in the techniques disclosed in Patent Documents 1 and 2, the temperature gradient of Hc can be made to be rapid to a certain extent at a low temperature equal to or lower than 350° C. However, to reduce the magnetization transition width, it is important to make the temperature gradient of the saturation magnetic field Hs rather than Hc. The saturation magnetic field is a magnetic field in which a loop is closed in a magnetization curve. This is a value corresponding to the inverting magnetic field of the particles most hardly causing the magnetization inversion out of the magnetic particles of the magnetic layer, that is, the maximum value of the inverting magnetic field. Accordingly, by making the temperature gradient of the saturation magnetic field rapid, a sharper magnetization transition is caused and thus excellent recording and reproducing characteristics are obtained.
The invention is made in view of the above-mentioned situations, and an object of the invention is to provide a thermal-assist magnetic recording medium which can accomplish a surface recording density of 1 Tbit/inch2 or more and a magnetic recording and reproducing apparatus having the thermal-assist magnetic recording medium.